By known methods liquid phases mixed with each other are principly separated either by distilling in processes where the boiling points of the phases adequately differ from each other, or by decanting in processes where the specific gravities of the phases are different.
The choice of the separation method is determined by various technical and economic factors and the separation capacity required of the process.
Decanting does not in general require heat energy and has no detrimental impacts on the properties of the separated liquid phases. It is primarily employed when the separation capacity required of the process can be achieved by it. Typical applications of the decanting method are separation of oil from waste waters and recovering turpentine-compounds from the condensates of cellulose mills.
The prior art decanters are either vertical or horizontal decanters or combinations of these.
In the most commonly employed vertical decanters, e.g. the Swedish Linder decanter, the decanter vessel is remarkably high compared to its diameter. The liquid to be decanted is supplied beneath the phase boundary from which point the lighter phase rises up to the surface and the heavier phase descends to the bottom. The operation principle is, however, disadvantageous for separation of micro drops of the lighter phase, as the drops which rise at a constant velocity determined by their viscosity and specific gravity ratios must travel a long way through the heavier phase and further, the downward flow of the heavier phase decreases the rising velocity of the drops. For these reasons the residence time in the vertical decanter must be long and the decanter vessel large in relation to the volume of the phases to be separated.
The dimensions of the known horizontal decanters differ from the ones described above, i.e. the area of the decanter vessel is large in relation to the height of the vessel. In a typical application the vessel is a long basin, into one end of which the phases are supplied. The phases flow in the longitudinal direction of the vessel and the heavy phase is discharged at the opposite end of the vessel. A horizontal decanter solves the problems of separation of the lighter phase microdrops but an essential drawback of the solution is the size of the decanter vessel and thus the large space it requires in industrial applications.
Reliable operation of a decanter in varying process conditions presupposes that the layer thickness of the lighter phase in the decanter exceeds a minimum which is dependent of the density ratio of the phases to be separated. Even in horizontal decanters this results in unreasonable and uneconomical dimensions which are difficult to apply in practice. Furthermore, a horizontal decanter with optimum dimensions is difficult to construct pressure-proof.
More advanced solutions are disclosed in U.S. Pat. Nos. 4,132,651 and 4,115,279 in which attempts have been made to improve the separation capacity by dividing the flow of the phases to be separated in several parallel flow passages whereby the theoretical separation capacity is improved in relation to the number of the flow passages. In these disclosures the problems arise in having an equal amount of flow divided in each of the adjacent flow passages. Exact division of the flow would presuppose a remarkably high flow resistance and thus a turbulent flow which, however, results in emulsification which in turn affects the operation of the decanter and decreases its separation capacity. Emulsification is especially problematic in decanting the turpentine-containing condensates of a cellulose mill.
A solution of the above described type is disclosed also in U.S. Pat. No. 4,357,241. The decanter vessel of this disclosure is circular and the flow in the adjacent passages takes place radially towards the periphery or the center of the vessel. However, the problems of dividing the flow and the the emulsification are not eliminated with this device.
A common feature of the known decanter constructions is that the lighter phase is removed through an overflow and the heavier phase through another overflow in such a way that height ratios of the phase boundary created in the vessel are determined by the height differences of the overflows.
In the known applications, the gases ended up in the vessel with the phases must rise up in bubbles through both liquid phases. This results in disturbances in the desired laminar flow and in certain conditions, detrimental emulsion may also be formed in the boundary surface of the phases.